Error when applying a function multiple times - c++

Question

I wrote a function to find all combinations of integers between two bounds. To do this, I wrote a function with the same name that finds all combinations of integers between two bounds of a certain size.
In main, I set up a loop to call this function multiple times. When it is ran more than once with bounds that are sufficiently far apart, it causes an error, that is, the code reaches the logic error in the first function.
I do not know why multiple passes in the while-loop causes a problem because the variables should be reset each time.
#include <vector>
#include <stdexcept>
#define VAR1 3
#define VAR2 8
bool nextCombination(std::vector<int> &combo, int numItems, \
int lowerBound, int upperBound) {
if (combo.empty()) { //This is the first take.
for (int i = 0; i < numItems; ++i) {
combo.push_back(lowerBound + i);
}
return true;
} else if (combo[0] >= upperBound - numItems) { //This cleans up.
combo.clear();
return false;
} else {
for (int i = 0; i < numItems; ++i) {
if (i >= numItems || combo[i]+1 != combo[i+1]) {
++combo[i]; //This extends the front of the stack.
return true;
} else { //This pushes the first part of the stack back.
combo[i] = lowerBound + i;
}
}
}
throw std::logic_error("There is an error with nextCombination.");
}
bool nextCombination(std::vector<int> &combo, int lowerBound, int upperBound) {
int numItems = combo.size();
if (numItems >= upperBound - lowerBound) {
combo.clear();
return false;
} else if (numItems == 0) {
combo.push_back(0);
return true;
} else {
if (nextCombination(combo, numItems, lowerBound, upperBound)) {
return true;
} else {
combo.clear(); //This line shouldn't be needed.
return (nextCombination(combo, numItems+1, lowerBound, upperBound));
}
}
}
int main() {
for (int i = 0; i < VAR1; ++i) {
std::vector<int> combo;
while (nextCombination(combo, 0, VAR2)) ;
}
return 0;
}

Answer 1

I don't understand the algorithm, but the problems are clear enough.
Step one, reduce the size of the problem I chose
#define VAR1 1
#define VAR2 2
Run the code, crash in operator[] here,
if (i >= numItems || combo[i]+1 != combo[i+1])
Values of variables
combo = vector of size 1
numItems = 1
i = 0
combo[i+1] is a subscript out of bounds error.
It doesn't take long to step through the code to get to this point. It happens immediately on the second iteration of the inner loop in main. Since I don't understand what your code is trying to do I can't suggest a fix. But hopefully the error is clearer to you now.
Since it seems you were unaware of this subscripting issue, you should change to using at instead of [] that way you get defined behaviour even on a subscript error.